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www.sciencedirect.comFull paper/MemoireEfficient and green oxidation of alcohols with tert-butyl
hydrogenperoxide catalyzed by a recyclable magnetic
core-shell nanoparticle-supported oxo-vanadium ephedrine
complex
Amin Rostami a, b, *, Omid Pourshiani a, Neda Darvishi a, Bahareh Atashkar a
a Department of Chemistry, Faculty of Science, University of Kurdistan, 66177-15175, Sanandaj, Iran
b Research Center for Nanotechnology, University of Kurdistan, P.O. Box 416, Sanandaj, Irana r t i c l e i n f o
Article history:
Received 20 May 2016
Accepted 18 August 2016
Available online 20 September 2016
Keywords:
Alcohol
Carbonyl compound
Oxidation
tert-Butyl hydrogenperoxide
Magnetic nanoparticles
VO(ephedrine)2 complex* Corresponding author. Department of Chemistr
University of Kurdistan, 66177-15175, Sanandaj, Iran
E-mail address: a_rostami372@yahoo.com (A. Ro
http://dx.doi.org/10.1016/j.crci.2016.08.003
1631-0748/© 2016 Académie des sciences. Publishea b s t r a c t
A novel method for the oxidation of alcohols to the corresponding carbonyl compounds
has been successfully developed using tert-butyl hydrogenperoxide (TBHP) in the presence
of a catalytic amount of recyclable magnetic nanoparticle-supported oxo-vanadium
ephedrine complex (VO(ephedrine)2@MNPs) in PEG as a green solvent at 80 C. The
catalyst can be magnetically recycled and successfully reused in six subsequent reaction
cycles with only slight decreases of its catalytic activity.
© 2016 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.1. Introduction
Recently, magnetic nanoparticles (MNPs) have been
used as a new alternative to porous materials for support-
ing catalytic transformations [1e7]. Their simple magneti-
cally driven separation from a liquid-phase reaction makes
catalyst recovery and recycling much easier than by cross-
flow filtration and centrifugation. Additionally, the MNP-
supported catalysts also show high dispersion and reac-
tivity with a high degree of chemical stability. By utilizing
these advantages of magnetic nanoparticles over other
supporting materials, various catalysts and ligands have
been immobilized on these nanoparticles [8e12].
The selective oxidation of primary and secondary alco-
hols to the corresponding carbonyl compounds is one of they, Faculty of Science,
.
stami).
d by Elsevier Masson SAS. Amost challenging reactions in organic chemistry [13],
because aldehydes and ketones are important intermediates
in the synthesis of other organic compounds and they are
utilizedparticularly in themanufactureofmedicines,flavors,
fragrances and aniline dyes [14]. Traditionally, oxidation of
alcohols has been performed with a stoichiometric amount
of metal oxidants, such as Cr(VI) salts [15], permanganate
[16] and bromate [17] reagents. However, these reagents
show poor atom efficiency and generate a large amount of
environmentally ill-disposed heavy-metal waste. Therefore,
catalytic oxidation methods, which employ a variety of
transitionmetals such asCu (I) [18], Ni (II) [19], Co (II) [20], Pd
(II) and manganese oxides [21e24] using molecular oxygen
[25,26], H2O2 [27] and TBHP [28,29] as oxidizing agents are
well explored. Vanadium based catalytic systems also have
shownpotential for alcohol oxidation, in some cases proving
effective for substrates where palladium catalysts display
limited activity [30e35]. However, the major disadvantage
of metal-based catalyst systems is their separation from thell rights reserved.
A. Rostami et al. / C. R. Chimie 20 (2017) 435e439436reaction solution by classical methods; they may possibly
leave toxic traces of heavy metals in the products. Also, the
reuse of the catalysts is difficult. These drawbacks can be
overcome by immobilization of these catalysts on MNPs.
2. Experimental
2.1. General procedure for the oxidation of alcohols to
carbonyl compounds
The alcohol (1 mmol) was added to a mixture of TBHP
(1 mmol) and VO(ephedrine)2@MNPs (50 mg) in PEG
(1 mL), and then the mixture was refluxed at 80 C for the
time specified. The progress was monitored by TLC (EtOAc/
n-hexane, 1/2). After completion of the reaction, the cata-
lyst was separated from the product by an external magnet
(within 5 s), and the mixture was washed with EtOAc
(25 mL) and decanted. The decanted mixturewas washed
with 30% NaOH (5mL) and the organic layer was dried over
anhydrous Na2SO4. The evaporation of EtOAc under
reduced pressure gave the pure products in 85e98% yields.
3. Results and discussions
In continuation of our studies on the synthesis and
application of magnetic nanocatalysts [36e39], herein, weHN
OHCH3
Si Cl
MeO
MeO
MeO
+
NaH
EtOH/H2
reflux
2 2
EtOH/H
reflux
SMN
Ph
CH3
CPTMS
Si N
CH3
O
N
H3C
Me
OMe
V
O
VO(ephedrine)2@MNPs
Ph
Ph
Si
O
O
O
O
OO
Fe3O4
NP
SiO2
Scheme 1. The preparation of
Table 1
The selective oxidation of benzyl alcohol to benzaldehyde using TBHP under dif
Entry Solvent T (C) Catalyst (mg
1 CH2Cl2 rt VO(ephedrin
2 CH3CN rt VO(ephedrin
3 H2O rt VO(ephedrin
4 PEG rt VO(ephedrin
5 PEG 80 VO(ephedri
6 PEG 80 None
7 PEG 80 VO(ephedrin
8 PEG 80 VO(ephedrin
9 PEG 80 Fe3O4 MNPs
a The bold letters represent the most effective reaction conditions.report the catalytic properties of VO(ephedrine)2@MNPs in
the selective oxidation of alcohols to carbonyl compounds
using TBHP as an oxidant. VO(ephedrine)2@MNPs were
synthesized according to our recently reported protocol for
the preparation of the chiral oxo-vanadium (þ)-pseudoe-
phedrine complex supported on magnetic nanoparticles
[38]. The process of the preparation VO(ephe-
drine)2@MNPs shown in Scheme 1(See the Supporting
information for experimental details). The characteriza-
tion of the catalyst was carried out by FT-IR, SEM, TGA and
EDX techniques.
The explanation of characterization techniques has been
provided in the Supporting information.
In order to optimize the reaction conditions, the
oxidation reaction of benzyl alcohol to benzaldehyde was
selected as a model reaction, and various parameters
including solvent, temperature and the amount of catalyst
were optimized to develop the scope of this reaction
further (Table 1). We have conducted the reaction in
different solvents such as CH2Cl2, CH3CN, H2O and PEG in
the presence of 50mg of the catalyst and 1mmol of TBHP at
room temperature (Table 1, entries 1e4), as shown in Table
1, in PEG, benzaldehyde was produced in 100% yield after
15 h. Therefore, PEG was chosen as the best reaction sol-
vent. When the reactionwas carried out at 80 C, the rate of
reaction was increased and the reaction was completedCO3
O (1:1),
, 24h
Si
MeO
MeO
MeO
N
OHCH3
VO(acac)2
2
Si
MeO
MeO
MeO
N
CH3
O
Si
MeO
MeO
MeO
N
Me
OMe
V
O
TMSP-ephedrine
VO(TMSP-ephedrine)2 Complex
EtOH/H2O (1:1),
reflux, 24h
2O (1:1),
, 24h
Ps
CH3
Ph
Ph
H3C
Ph
VO(ephedrine)2@MNPs
ferent conditions.
) Time (h) Conversion (%)
e)2@MNPs (50) 24 No Reaction
e)2@MNPs (50) 24 No Reaction
e)2@MNPs (50) 24 No Reaction
e)2)@MNPs (50) 15 100
ne)2@MNPs (50) 3 100a
24 20
e)2@MNPs (30) 8 100
e)2@MNPs (40) 7 100
(50) 15 50
Table 2
VO(ephedrine)2@MNPs (50 mg) catalyzed selective oxidation of alcohols (1 mmol) to carbonyl compoundsa using TBHP (1e4 mmol) as oxidant in PEG as
solvent at 80 C.
Entry Alcohol Time (h) TBHP (mmol) Producta Yield (%)b
1 3 1 98
2 2 1 96
3 47 1 97
4 4:20 4 95
5 6 4 91
6 7 4 98
7 10 4 98
8 55 1 90
9 50 1 85
10 15 4 92
11 4:50 4 94
12 3:15 4 96
13 7:15 4 93
14 12 4 90
a All products are known and were characterized by IR and 1H NMR as compared with those reported in the literature [25,26].
b Isolated yield.
A. Rostami et al. / C. R. Chimie 20 (2017) 435e439 437
V
O
N O
N O
Ph
Ph
tBuOOH
tBuOOH
R1 R2
O
+tBuOH
VO(ephedrine)2@MNPs
R1 R2
OH
MNPs
MNPs
V
O
O
N
O
O O
N
H
R2
R1
O
tBu
Ph
Ph
V
O
O
N O
O
N
Ph
Ph
O
tBu
V
O
O
N
O
O
N
Ph
Ph
Scheme 2. Proposed mechanism.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Co
nv
er
sio
n 
%
Run Number
Fig. 1. The recycling experiment of VO(ephedrine)2@MNPs (50 mg) in
oxidation of benzyl alcohol using TBHP in PEG at 80 C for 3 h.
A. Rostami et al. / C. R. Chimie 20 (2017) 435e439438after 3 h (Table 1, entry 5). To optimize the amounts of
catalyst, different amounts of VO(ephedrine)2@MNPs (0, 30
and 40 mg) were used in the oxidation of benzyl alcohol
using TBHP (1 mmol) in PEG at 80 C. In the absence of a
catalyst, the reactionwas incomplete even after 24 h (Table
1, entry 6). When 30 and 40 mg of catalyst were used, the
reaction times were prolonged to 8 and 7 h under same
reaction conditions respectively (Table 1, entries 7 and 8). It
should be noted that the use of Fe3O4 MNPs instead of
VO(ephedrine)2@MNPs gives a lower yield of benzaldehyde
even after prolonging the reaction time (Table 1, entry 9).
As a result, the optimized reaction conditions were iden-
tified as using 50 mg of VO(ephedrine)2@MNPs in PEG as a
solvent at 80 C.
In order to generalize the scope of the reaction, a series
of alcohols was subjected to oxidation under the optimized
reaction conditions, the results are presented in Table 2. As
shown in Table 2, various types of primary benzylic alco-
hols, including those with both electron-withdrawing and
electron-donating groups, were selectively converted to
the corresponding carbonyl compounds in excellent yields
under optimal reaction conditions (Table 2, entries 1e6).
Interestingly, 2-methyl benzyl alcohol as a model for hin-
dered primary benzylic alcohols was also successfully
oxidized to its corresponding carbonyl compound,
although a longer reaction time was required (Table 2,
entry 3). The primary benzylic alcohols with electron-
withdrawing groups show less reactivity and their oxida-
tion requires some more of TBHP (4 mmol) (Table 2, entries
4e6). The oxidation of 2-phenyl ethanol as a model for
primary aliphatic alcohols was satisfactorily subjected as
well (Table 2, entry 8). In general, alcohols containing
heterocyclic moiety are highly challenging substrates for
oxidation in most transition-metal catalyst systems
because they have a tendency to bind to transition metals
and can act as catalyst deactivators. Another important
aspect of this method is the successful oxidation of furfuryl
alcohol and 3-pyridinemethanol to give the expected al-
dehydes (Table 2, entries 9 and 10). Furthermore, benzylic
and cyclic secondary alcohols could be effectively oxidized
using the present method to give excellent yields of the
corresponding carbonyl products (Table 2, entries 11e14).
On the basis of a previously reported mechanism for the
oxidation of alcohols using peroxides (H2O2 or TBHP) in the
presence of vanadium based catalysts, one explanation for
this process is that the mechanism involves radical in-
termediates [30]. Another explanation is that the oxidation
process involves the reaction of the TBHP with VO(ephe-
drine)2@MNPs to form an active peroxidovanadium (V) in-
termediate, which subsequently oxidizes the substrate
molecule and returns to the original state [31,32]. In order to
conduct mechanistic investigation, the oxidation reaction of
benzyl alcohol using tBuOOH as an oxidant and VO(ephe-
drine)2@MNPsasa catalystwas carriedout in thepresenceof
2,20-azobis(isobutyronitrile) as a radical trap. Itwasobserved
that addition of 2,20-azobis(isobutyronitrile) has no signifi-
cant effect on the yield of the product and reaction time. This
observation suggests that the mechanism probably involves
the TBHP activation by vanadium (V) centers forming a
peroxy intermediate (Scheme 2). However, at this time theexact mechanism of the reaction is not clear and the actual
role of this catalyst should be further studied in detail.
For practical purposes, the ability to easily recover and
recycle the catalyst is highly desirable. The reusability of
this magnetic nanocatalyst was examined using benzyl
alcohol as a model substrate. After the first use of the
catalyst in the oxidation of benzyl alcohol to give benzal-
dehyde, the catalyst was rapidly separated by an external
magnet and was washed thoroughly with EtOAC. It was
reused for subsequent experiments under similar reaction
conditions. As shown in Fig. 1, the catalyst can be recycled
up to six runs without significant loss of activity.
Table 3
Comparison of efficiency of various supported vanadium based catalysts in the oxidation of benzyl alcohol.
Entry Catalyst (mg) Conditions Time (h) Yield (%) Ref.
1 VO(ephedrine)2@MNPs (2 mol %) TBHP (1 mmol), PEG, 80 C 3 98 This work
2 VO(ephedrine)2 (2 mol %) TBHP (1 equiv), PEG, 80 C 1.5 95 This work
3 VO(Schiff base)@nano-starch (5 mol %) TBHP (1.5 equiv), MeCN, 65 C 1 92 32
4 VO(acac)2@Polyaniline (2.3 mol %) O2, toluene, 100 C 9 98 33
5 VO(Schiff base)@ nano silica (20 mol %) H2O2 (1.2 equiv), MeOH, reflux 4 95 34
6 VO(Schiff base)@PMOs (2 mol %) TBHP (1.5 mmol), MeCN, 65 C 3 90 35
A. Rostami et al. / C. R. Chimie 20 (2017) 435e439 439We have compared the efficiency of VO(ephe-
drine)2@MNPs with VO(ephedrine)2 (the homogenous
analogue) and other reported heterogeneous vanadium
based catalysts in the oxidation of benzyl alcohol (Table 3).
It was observed that the reaction proceeds more rapidly
with a homogenous analogue than VO(ephedrine)2@MNPs,
instead, a higher yield has been obtained by VO(ephe-
drine)2@MNPs, suggesting the importance of magnetic
separation (Table 3, entries 1 and 2). The characteristic
aspects of the described method in comparison with other
previously reported catalysts are: (1) the product separa-
tion and catalyst recycling are easier and simpler with the
assistance of an external magnet and (2) the used solvent
(PEG) is green.
4. Conclusion
VO(ephedrine)2@MNPs as a magnetically separable and
efficient nanocatalyst were applied for the selective
oxidation of alcohols to aldehydes and ketones using TBHP
as an oxidant in good to high yields in PEG as a green sol-
vent at 80 C. The product separation and catalyst recycling
are easier and simpler with the assistance of an external
magnet. The catalyst can be recycled and reused six times
with little loss of activity.
Acknowledgements
We are grateful to the University of Kurdistan Research
Councils for partial support of this work.
Appendix A. Supplementary data
Supplementary data related to this article can be found
at http://dx.doi.org/10.1016/j.crci.2016.08.003.
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Sommaire / Contents

Artificial photosynthetic antennas and reaction centers

Continuous and reactive nanocrystallization: New concepts and processes for dual-use advances

Three-component synthesis of novel spirooxindole–furan derivatives using pyridinium salts

New <i>p</i>-aminophenol-based dendritic melamines. Iterative synthesis, structure, and electrochemical characterisation

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